A shock wave propagated through the Earth as a result of an earthquake. There are four types of wave. P (compressional) and S (transverse) waves both have high frequency, and are transmitted through the Earth, but only P waves can travel through fluid zones. L waves are transverse, have low frequency, and are confined to the upper part of the crust. Rayleigh waves develop close to the epicentre, and are responsible for the rolling movement of the Earth's surface during an earthquake.
A seismic wave is a wave that travels through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion. Seismic waves are also continually excited by the pounding of ocean waves and the wind.
Types of seismic wave
There are two types of seismic wave, namely, 'body wave' and 'surface wave'.
Body waves
Body waves travel through the interior of the Earth.
P waves
P waves are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. These waves generally travel slightly less than twice as fast as S waves and can travel through any type of material. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. P waves are sometimes called "primary waves", and are not as destructive as the S waves and surface waves that follow them.
S waves
S waves are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S waves are sometimes called "secondary waves", and are several times larger amplitude than the P waves.
Surface waves
Surface waves are analogous to water waves and travel just under the Earth's surface. They travel more slowly than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most destructive type of seismic wave. There are two types of surface waves: Rayleigh waves and Love waves.
Rayleigh waves
Rayleigh waves, also called ground roll, are surface waves that travel as ripples similar to those on the surface of water. They are slower than body waves and, according to legend, can readily be seen during an earthquake in an open space like a parking lot where the cars move up and down with the waves.
Love waves
Love waves are surface waves that cause horizontal shearing of the ground. They usually travel slightly faster than Rayleigh waves.
P and S waves in Earth's mantle and core
When an earthquake occurs, seismographs near the epicenter, out to about 90° distance, are able to record both P and S waves, but those at a greater distance no longer detect the S wave. The Moon has been proven by seismic testing to have a solid core, because it conducts shear waves.
Some principles of locating an event
In the case of local or nearby earthquakes, the difference in the arrival times of the P and S waves can be used to determine the distance to the event.
A quick way to determine the distance from a location to the origin of a seismic wave less than 200 km away is to take the difference in arrival time of the P wave and the S wave in seconds and multiply by 8 kilometers per second.
At teleseismic distances, the first arriving P waves have necessarily travelled deep into the mantle, and perhaps have even refracted into the outer core of the planet, before travelling back up to the Earth's surface where the seismographic stations are located. The waves travel more quickly than if they had traveled in a straight line from the earthquake. Density in the planet increases with depth, which would slow the waves, but the modulus of the rock increases much more, so deeper means faster. Since P waves move at many kilometers per second, being off on travel-time calculation by even a half second can mean an error of many kilometers in terms of distance.
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